It is often desirable to apply clear, pigmented or dyed ink coatings or layers to form distinctive logos or production prints on game balls (e.g., golf balls, ping pong balls, billiard balls, baseballs, basketballs, racquet balls, handballs, etc.). Various commercially available inks are commonly used for this purpose.
To clarify the difference between logo printing and production printing, a brief description of these processes as they are applied to game balls such as golf balls (e.g., having curved surfaces) is provided below. Golf balls are commonly one-piece, two-piece or three-piece constructions. One-piece balls are made from a homogeneous polymer shaped into a golf ball. Two-piece golf balls comprise an inner core and an outer surrounding polymeric shell. Three-piece golf balls comprise various combinations of a core (wound or unwound), one or more intermediate polymeric shells and an outer polymeric cover. The cover polymer used in two-piece and three-piece balls may, for example, be balata, an ionomeric polymer (e.g., SURLYN.RTM.) or a polyurethane.
Golf ball covers are commonly painted with a primer coat which may be colored (e.g., white) or transparent. Alternately, the cover itself may contain a colorant. Typically, a tough, often glossy, topcoat is applied over the cover and/or the primer coat to form a protective outer seal on the golf ball. The topcoat may comprise, for example, a two component urethane. The topcoat typically increases the shine (i.e., glossy appearance) and durability of the golf ball to enhance or brighten its appearance.
As used herein, "production printing" refers to a process wherein ink is applied directly to the cover or to the primer coat and the ink is then further coated with a topcoat. The image produced thereby is a "production" print and the ink used for this purpose is a "production" ink. In production printing, for some applications, when ink is applied directly to a cover, the cover surface is first prepared for bonding, for example, by sandblasting, plasma treatment or corona bonding, to enhance the bond between the ink and the cover. Thereafter, the ink is applied to the roughened cover. A transparent water based or solvent based overcoat may be applied over the ink layer and on the roughened cover to smooth out the cover and ink surfaces. Examples of such overcoats include urethane, polyester and acrylic. Thereafter, a topcoat is preferably applied to the overcoat.
Alternatively, "logo printing" as also used herein, involves the application of the ink directly onto a topcoat. The image produced thereby is a "logo" and the ink is a logo (or custom) ink. Thus, by use of production and/or logo printing one may add decorative markings such as a company trademark, symbol or the like to increase brand recognition and/or to enhance the appearance and/or the visibility of golf balls, game balls and the like. As used herein, the term "ball" is used to refer to game balls, golf balls and the like.
Most commonly, logos and production prints are applied to golf balls by a pad printing process and apparatus. Pad printing uses an etched image plate (i.e., a cliche) having a negative etching of the desired image. The image plate, typically, is made of a tough material such as metal, steel, other alloy or photopolymer which normally has a uniform thickness except for the area defining the negative etched image. The plate may optionally be coated with one or more protectant layers or materials, to enhance its useful life. Typically, the depth of the etched image is from about 5 microns to about 30 microns or any value therebetween.
During pad printing, ink is applied to the image plate, thus filling the etched image. Excess ink is then scraped off of the image plate, leaving behind ink only within the etched image. A printing pad is then momentarily lowered and pressed onto the inked image plate to lift ink off of the etched ink filled cavity onto the printing pad. The ink so lifted defines the shape of the etched image. The inked pad is then momentarily lowered and pressed onto, for example, a golf ball, thereby releasing the ink from the pad to the golf ball. The ink released from the pad forms, on the spherical surface of the ball, an image corresponding to that of the etched cavity.
This process of inking the image plate, scraping off excess ink, lifting off ink onto the printing pad and releasing the ink from the pad to the object (e.g., golf ball) to be inked may be repeated to print a plurality of images on a plurality of types of balls with various inks having desirable ink properties. The process of pad printing is well known. See, for example, U.S. Pat. No. 5,513,567 (Froh et al.); U.S. Pat. No. 4,896,598 (Leech, Jr.); U.S. Pat. No. 4,803,922 (Denesen); U.S. Pat. No. 4,745,857 (Putnam et al.); and U.S. Pat. No. 5,237,922 (Ho).
Printing pads are made from a resilient material such as silicone rubber which desirably picks up ink from the etched cavity of the image plate during lift-off and releases all of the ink lifted off when brought into contact with the article to be printed. Once the ink is deposited, it is cured, most commonly by a thermal curing process.
However, during manufacturing of printed articles such as game balls and golf balls, ink transfer problems are often encountered. For example, while it is desirable that all of the ink picked up by the printing pad be fully released onto the article to be printed, sometimes complete release is not achieved. Consequently, subsequent articles to be printed upon by the same printing pad member may have excessive ink or misaligned ink deposited thereon. Such improper ink deposition leads to unwanted ink contamination of balls, either directly between balls or indirectly by first transferring ink to ball handling equipment or both. Resolution of such problems requires expensive positioning equipment to prevent unwanted contact between balls, between ink depositing members and balls and between ball handling equipment and balls, respectively.
To overcome such ink transfer problems, intermediate thermal curing steps are introduced into the manufacturing process. There are several disadvantages to thermal curing, however. These include (1) high energy consumption, (2) long cooling cycles, (3) restricted material selection to thermally curable and thermally stable polymeric materials and (4) use of costly ventilating systems to dissipate vapors generated during thermal curing.
In addition to the problems associated with ink transfer before the ink is cured, post manufacturing problems are also commonly encountered even after curing takes place. Ink layers, after cure, may not possess a desirable level of adhesion to a substrate article surface. For example, a logo printed onto a golf ball topcoat is subjected to repeated "hard" impacts by a golf club during the golf ball's normal useful life. If adhesion, toughness, flexibility and/or hardness are at an undesirable level, ink deposited upon a topcoat (e.g., a logo printed upon a finished ball having an underlying topcoat or a clear coat) and/or ink deposited under a topcoat (e.g., a production print formed by the ink layer being interposed between the cover or primer coat and an overcoat and/or a topcoat) will abrade, flake, crack or otherwise separate from the golf ball topcoat, overcoat, the cover and/or the primer coat. After repeated impacts, such lack of adhesion, toughness, flexibility and/or hardness yields an unsightly golf ball.
The adhesion of the ink to the ball is in turn affected by the sufficiency of the curing of the ink. When the ink is not properly cured, it tends to detach from the surface of the golf ball. Separation of UV curable ink from the ball is more likely when the ink or ink layer is thicker because the ink is less likely to be completely through-cured, i.e. cured through the entire thickness of the UV curable ink coating.
Also, UV curable inks which contain certain colored pigments are more prone to insufficient curing, even when UV radiation or light is used to cure the coating. Pigments, particularly those which impart the colors black, blue, green or white to the ink, absorb the UV light which is necessary to activate the UV photoinitiators which initiate the curing of the ink. Specifically, these pigments and the UV photoinitiators absorb UV light having the same UV wavelengths. Because both the pigments and the UV photoinitiators are competing for the same UV radiation, some of the UV photoinitiators in the ink, particularly those below the ink layer's outer surface will not absorb enough energy needed to initiate the curing in the lower portions of the ink layer. Also, the UV photoinitiators at the surface of the ink absorb the UV light, thereby limiting the amount of UV light that penetrates to lower depths in the ink layer to permit curing of the ink layer at the substrate interface. As a result, insufficient curing often occurs. To remedy such deficiency, more energy and/or longer curing times can be used. However, increases in energy doses and curing time is not desirable since it leads to inefficient production of golf balls as well as potentially adverse effects on the golf ball materials. For example, to increase the curing energy, the line speed, i.e., the speed by which the balls are moved toward the curing energy source, must be reduced. This decrease in line speed tends to melt or sink the cover on the golf balls due to the heat output of the curing source. Therefore, there exists a need for a durable ink for golf balls which thoroughly cures upon exposure to UV/vis light without exposing the substrate to greater energy.
To overcome these and other drawbacks, inks used in production and logo printing must have sufficient durability. Durability is influenced by such factors as ink layer flexibility (i.e., ink layer brittleness), ink layer resistance to abrasion, ink layer hardness, adhesion to golf ball cover polymers such as ionomers (e.g., SURLYN.RTM.), balata, polyurethane, polyolefin mixtures thereof, adhesion to topcoats, adhesion to primer coats and intercoat adhesion between various layers of inks and/or other overcoats and/or topcoats.
To overcome these and other problems, novel inks (1) that are radiation curable and have a high cure rate (i.e., require low curing time) (2) that require minimal cooling, if any, (3) that are almost instantaneously cured, (4) that permit use of an extensive color palette for the pigments added to the ink and (5) that permit the use of a broader range of polymeric core, intermediate shell (or windings) or cover materials for use with golf balls and game balls are sought. These requirements are met by radiation curable inks which are the subject of the present invention.